1618 IEEE TRANSACTIONS ON MEDICAL IMAGING, VOL. 37, NO. 7, JULY 2018 Repeatability Assessment of Intravascular Polarimetry in Patients Martin Villiger , Member, IEEE , Kenichiro Otsuka, Antonios Karanasos, Pallavi Doradla, Jian Ren , Norman Lippok, Milen Shishkov, Joost Daemen, Roberto Diletti, Robert-Jan van Geuns, Felix Zijlstra, Jouke Dijkstra, Gijs van Soest, Evelyn Regar, Seemantini K. Nadkarni, and Brett E. Bouma Abstract — Intravascular polarimetry with polarization sensitive optical frequency domain imaging (PS-OFDI) measures polarization properties of the vessel wall and offers characterization of coronary atherosclerotic lesions beyond the cross-sectional image of arterial microstructure available to conventional OFDI. A previous study of intravascular polarimetry in cadaveric human coronary arteries found that tissue birefringence and depolarization provide valuable insight into key features of atherosclerotic plaques. In addition to various tissue components, catheter and sample motion can also influence the polarization of near infrared light as used by PS-OFDI. This paper aimed to evaluate the robustness and repeatability of imaging tissue birefringence and depolarization in a clinical setting. 30 patients scheduled for percutaneous coronary interven- tion at the Erasmus Medical Center underwent repeated Manuscript received November 24, 2017; revised March 4, 2018; accepted March 5, 2018. Date of publication March 21, 2018; date of current version June 30, 2018. This work was supported in part by the National Institutes of Health under Grant P41EB-015903 and Grant R01HL-119065, and in part by Terumo Corporation. The work of K. Otsuka was supported in part by the Japan Heart Foundation and in part by the Bayer Yakuhin Research Grant Abroad. (Corresponding author: Martin Villiger.) M. Villiger, K. Otsuka, P. Doradla, J. Ren, N. Lippok, M. Shishkov, and S. K. Nadkarni are with the Wellman Center for Photomedicine, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114 USA (e-mail: mvilliger@mgh.harvard. edu; kotsuka@mgh.harvard.edu; pdoradla@mgh.harvard.edu; jren@mgh.harvard.edu; nlippok@mgh.harvard.edu; shishkov@helix. mgh.harvard.edu; snadkarni@mgh.harvard.edu). A. Karanasos was with the Department of Interventional Cardiology, Thoraxcenter, Erasmus Medical Center, Rotterdam, The Netherlands. He now is with the First Department of Cardiology, Hippokration Hospital, University of Athens, Athens, Greece (e-mail: akaranasos@ hotmail.com). J. Daemen, R. Diletti, R.-J. van Geuns, F. Zijlstra, and G. van Soest are with the Department of Interventional Cardiology, Thoraxcenter, Erasmus Medical Center, Rotterdam, The Nether- lands (e-mail: j.daemen@erasmusmc.nl; r.diletti@erasmusmc.nl; r.vangeuns@erasmusmc.nl; f.zijlstra.1@erasmusmc.nl; g.vansoest@ erasmusmc.nl). J. Dijkstra is with the Department of Radiology, Leiden University Medical Center, Leiden, The Netherlands (e-mail: j.dijkstra@lumc.nl). E. Regar was with the Department of Interventional Cardiology, Thorax- center, Erasmus Medical Center, Rotterdam, The Netherlands. She now is with the Heart Center, University Hospital Zurich, Zürich, Switzerland (e-mail: evelyn.regar@usz.ch). B. E. Bouma is with the Wellman Center for Photomedicine, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114 USA, and also with the Institute for Medical Engineering and Science, Massachusetts Institute of Technology, Cambridge, MA 02142 USA (e-mail: bouma@mgh.harvard.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMI.2018.2815979 PS-OFDI pullback imaging, using commercial imaging catheters in combination with a custom-built PS-OFDI console. We identified 274 matching cross sections among the repeat pullbacks to evaluate the reproducibility of the conventional backscatter intensity, the birefringence, and the depolarization signals at each spatial location across the vessel wall. Bland–Altman analysis revealed best agreement for the birefringence measurements, followed by backscatter intensity, and depolarization, when limiting the analysis to areas of meaningful birefringence. Pearson correlation analysis confirmed highest correlation for birefringence (0.86), preceding backscatter intensity (0.83), and depolarization (0.78). Our results demonstrate that intravascular polarimetry generates robust maps of tissue birefringence and depolarization in a clinical setting. This outcome motivates the use of intravascular polarimetry for future clinical studies that investigate polarization properties of arterial atherosclerosis. Index Terms— Endoscopy, evaluation and performance, heart, optical imaging, optical coherence tomography, polarimetry, validation, vessels. I. I NTRODUCTION I NTRAVASCULAR optical coherence tomography (OCT) and optical frequency domain imaging (OFDI), a second- generation implementation of OCT, currently offer the highest spatial resolution for invasive coronary imaging. Visualizing the detailed plaque microstructure has helped to advance our understanding of the pathogenesis of coronary artery disease [1], [2] and has offered new strategies to guiding percu- taneous coronary interventions in clinical practice [3], [4]. The high spatial resolution has enabled investigation of fibrous cap morphology in plaque disruption [5]–[7] and erosions [8], the two major pathways to acute coronary events. It also offered insight into macrophage accumulation [9], [10], considered an important contributor to plaque instability. Despite the merits of contemporary intravascular imaging, there remains a need for improved imaging methods to furnish novel insights into the mechanisms of thrombotic complications, and to evaluate the effects of therapeutic interventions. Combin- ing OCT with the superior imaging depth of intravascu- lar ultrasound (IVUS) would enable evaluation of plaque burden together with microstructural details [11]. Fluores- cence, from endogenous origin or injectable imaging probes offers an interesting avenue to complement OCT and enhance plaque characterization [12]–[16], but requires custom mul- timodal imaging catheters. We have previously reported on 0741-3106 © 2018 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.